To minimize the amount of voice information transmitted over a wireless communication network, and thus maximize the number of phone calls supportable on the network at any one time, cellular phones utilize voice coders/decoders, or codecs. Codecs remove much of the redundant or unnecessary information from a speech signal. Then the fundamental elements of the speech are transmitted over the network to a receiving cellular phone where they are decoded, or recombined with data that resembles the previously removed information. This results in reconstituted speech that can be recognized by the end user. The codecs must balance the need for minimal data transmission with the need to retain enough of the original speech information to sound natural when decoded on the receiving end. In general voice codecs today can compress speech signals to between 4.5 k-8 k bits per second, with 2.4 k bits per second being roughly the minimal rate required to maintain natural-sounding speech. Despite the ability to compress speech to these low bit rates, the network infrastructure for handling large volumes of voice calls is limited in many markets, particularly in emerging markets in developing countries. This can make the cost of a wireless phone call there significant.
An alternate and increasingly popular method of communicating via cellular phones is text messaging. In response to the high costs of voice calls, text based mobile-to-mobile messaging called SMS, or Short Message Service, has become heavily used in some markets, particularly amongst younger demographics. SMS enables a user to transmit and receive short text messages at any time, regardless of whether a voice call is in progress. The user typically types in the message text through the small keyboard that is provided on the device. The messages are hardware limited to 160 characters, and are sent as packets through a low bandwidth, out-of-band message transfer channel. This allows for facile communication with minimal burden on the wireless network.
Most legacy wireless network systems such as GSM, TDMA, and CDMA have a text/data channel capable of sending and receiving SMS, so the infrastructure for this service already exists even in emerging markets in developing countries. Some estimates now place the global number of SMS messages at nearly 40 billion messages per month. It is thought that SMS is now the most significant source of non-voice based revenue to wireless network operators worldwide. As a result carriers are very interested in promoting the use of SMS. Indeed, network operators in developing markets may limit the implementation of more advanced voice network infrastructures due to the large revenues associated with text messaging.